Magnetic resonance imaging is imaging procedure which reconstructs an image from the MR signal generated by exciting magnetically the nuclear spin of the object placed in the static magnetic field with the RF signal of Larmor frequency.
In the magnetic resonance imaging, physical quantities, such as an apparent diffusion coefficient (ADC) and a fractional anisotropy (FA) in the diffusion weighted imaging (DWI), a longitudinal relaxation (T1) value, a transverse relaxation (T2) value, proton density, temperature, the amount of chemical shifts, blood flow volume, oxygen concentration and so forth, can be measured, and improvement of measurement accuracy of these physical quantities has been required strongly.
In particular, the DWI is often used as a functional imaging method which can image an infarction part or a cancer part. In the DWI, the diffusion behavior of a molecule is imaged by applying a MPG (motion probing gradient) pulse, using the fact that the phase of a proton changes with the diffusive motion of a molecule. As a sequence for the DWI, EPI (echo planar imaging) sequence is used widely.
In the DWI, a lesion part, such as an infarction part or a tumor part, and normal parts can be distinguished according to the degree of diffusion.
However, since a T1 component and a T2 component are mixed into an image of the DWI, it is said to be difficult to determine an exact symptom from the image of DWI. Thus, parameters called ADC and FA, which are quantitative parameters showing only a diffusion effect, are used in many cases.
For example, the ADC value of a cerebral infarction part in an acute stage or a tumor part may become lower than the ADC value in a normal part in many cases. When screening of cancer is conducted by DWI, a quantitative image, such as an image of ADC or an image of FA, are generated from, for example, a wide range of volume data of the whole body (for example, refer to non-patent literature, “Takahara T, Imai Y, Yamashita T, Yasuda S, Nasu S, Van Cauteren M., Diffusion weighted whole body imaging with background body signal suppression (DWIBS): technical improvement using free breathing, STIR and high resolution 3D display. Radiat Med. 2004 July-August; 22(4):275-82”).
An image of ADC is generated from two or more images of DWIs, each corresponding to a different b value.
On the other hand, an image of FA is generated from two or more images of DWIs produced by applying MPG pulses in at least 6 axial directions which are different from each other to obtain b values. Here, the b value indicates an amount of the signal attenuation by diffusion.
As an index showing malignancy of cerebral infarction or cancer, quantification and standardization of ADC or FA, have been sought after.
Meanwhile, there is a technique which standardizes the display of an index indicating malignancy of lesion parts, such as cerebral infarction or cancer, only with an image of DWI. In this technique, an image which expresses the index is corrected, using a signal collected from specific regions, such as thalamus, which are considered to be a normal part in a head, as a reference value.
However, as mentioned above, the image of DWI does not necessarily include only quantitative parameters which indicate a diffusion effect. That is, T1 component and T2 component are mixed into the image of DWI.
In addition, the size of the imaging region and signal strength of the image of DWI differ from an object to another.
As a result, the index obtained simply from the image of DWI may include an error, and may not exactly show the malignancy of a lesion part.
Further, when an image of DWI is obtained by an EPI sequence, the image of DWI may cause a distortion or an error of signal strength due to an eddy current yielded by a MPG pulse with a strong gradient magnetic field or nonlinearity of a gradient magnetic field.
As a result, the ADC value and FA value calculated based on the image of DWI having such a distortion or an error of signal strength will also be shifted from true values, depending on tissues.
In addition, the error of the ADC value or FA value has a spatial distribution, i.e., a spatially non-uniform distribution.
When imaging a tissue in which a signal value is small while ADC is large, there is also a problem that a signal value becomes large due to a noise and causes a calculation error.
Furthermore, an error similar to the above-mentioned error produced in the image of ADC or FA may be produced also when quantitatively measuring other physical quantity by MRI.
The error in measuring quantitatively a physical quantity varies depending on imaging conditions, such as which equipment is used, what type of equipment is used, and when the imaging is performed, resulting in an impairment of diagnosis.
Therefore, even for different imaging conditions, such as an individual property of the equipment, a type of the equipment, and an imaging timing, it is necessary to stably acquire a diagnosing image without distortion to measure quantitatively physical quantity in a simple manner and with high precision.